Stir the flour, cornmeal, and salt together in a medium bowl. Scatter the butter on top and then quickly rub it in with your fingers until the mixture resembles coarse meal with lumps ranging from sand clump to pea size. Place the butter-flour mixture in the refrigerator while you measure and whisk together the yogurt, lemon juice, and cold water. Then, remove the butter-flour mixture from the refrigerator, make a well in the middle, and add the wet ingredients. Stir until this comes together into a messy ball, then gather it together with your hands, pat it into a thick disk, wrap tightly in plastic wrap and refrigerate for at least 1 hour and up to 2 days.

Heat the oil in a large sauté pan over medium-high heat, then fry the sausage, gradually breaking it into small chunks, until it is cooked through. Transfer this to a bowl and pour any accumulated grease out of the pan. Return the pan to the heat and add the butter. When the butter is foaming, stir in the chard and the maple syrup and a sprinkling of salt and pepper. Cook, stirring, until the chard is wilted, about 5 minutes. Taste and add more salt and pepper to taste, then stir this in with the sausage. Add all but about 1 tablespoon of the whisked eggs to the sausage and chard mixture, and stir everything together. (Don't worry about the eggs being raw -- they will bake in the oven!) Heat your oven to 375° F. Take out your galette dough and divide it into 6 equal portions, rolling each into a ball. Roll each ball out on a floured surface into a circle that's about 1/8 inch thick. Place each dough circle on a parchment-lined rimmed baking sheet. Spoon a generous scoop -- around 1/3 cup -- of the filling mixture into the center of one of the circles, then fold up the crust around it, pinching it and pleating it around the galette. Repeat with the rest of the dough circles. Brush the last of the whisked egg on the galette crusts, then stick them in the oven. Bake for 25 to 30 minutes until the crusts are brown and golden. Remove and cool slightly before serving, or cool completely and store in the refrigerator. Reheat in the oven or a toaster oven as needed before serving.